Assessing the ecological importance of climate regime shifts: An approach from the North Pacific Ocean

We used an indicator approach to address outstanding questions concerning the ecological importance of low-frequency climate variability in the northeast Pacific Ocean. Our data consist of a previously-published set of 33 climate and 64 biology time series, updated by us for the years 1965-2011 (for climate data) and 1965-2008 (for biology data). A model-selection approach showed that the first axis of variability in large-scale climate indices (PC1ci), the first and second axes for local climate parameters (PC1cp and PC2cp) and the second axis for biological variability (PC2biol) all showed temporal variability best described by abrupt shifts. In contrast, PC1biol showed gradual, rather than abrupt, temporal variability, suggesting that the leading axis of biological variability was not dominated by abrupt transitions following climate regime shifts. The leading mode of variability in detrended North Pacific sea surface temperature, the Pacific Decadal Oscillation, showed reduced amplitude from the late 1980s until the mid-2000s, and we found that this change in PDO behavior was associated with a decline in the strength of the leading pattern of basin-scale biological variability (PC1biol). A reversion to a PDO-negative state in the winter of 2007/08 was associated with the largest observed annual change in the PC1bio-PC2bio phase space, suggesting renewed ecological importance of the PDO. However, a subset of biology time series (n = 23) for which more recent data were available did not show persistent change in PC1bio or PC2bio during 2008-2011, thus failing to support the hypothesis of widespread ecological response to the putative 2007/08 shift. To further assess the possible ecological importance of low-frequency climate variability in recent years, we compared changes in the PDO-North Pacific Gyre Oscillation (NPGO) phase space for 2007/08 with ecologically important (1976/77) and less important (1988/89) climate regime shifts of the past. We found that all three shifts involved PDO-NPGO variability of similar magnitude (i.e., similar pulse disturbances), but that the 1976/77 shift was followed by a period of stability in a new climate state (i.e., strong press disturbance), while the 1988/89 shift was not followed by a period of stability (weak press disturbance). Data through 2013 suggest that the press disturbance following 2007/08 is similar to that following 1976/77, implying that the putative 2007/08 shift may eventually prove to be ecologically important. Our “pulse-press” approach provides a formal framework for distinguishing transient and persistent climate perturbations at the ends of time series.